The invention relates to a cutting tool having a plurality of indexable clamped-on "throwaway" tips or inserts releasably secured to an end of a shank or tool body thereof.
Conventional cutting tools with indexable clamped-on "throwaway" tips or inserts are known in shapes of end mill cutting tools as shown in FIGS. 16, 17, and 18.
The end mill cutting tool as shown in the aforesaid figures comprises a tool body 10 of a substantially cylindrical shape having two insert mounting recesses 12 formed at the front end thereof. A threaded hole 16 is formed in a mounting face 14 at the bottom of each of the mounting recesses 12 inwardly into the tool body 10. The cutting tool also includes two inserts 20 of a substantially square shape in plan view, having each thereof disposed on the mounting face 14. Each of the inserts 20 has a mounting hole 34 formed therethrough, through which a clamping screw 30 is screwed into the threaded hole 16, whereby a conical screw head 32 comes into close contact with a tapered face 36 of the mounting hole 34 of the insert 20, and each of the inserts 20 is secured pressed onto the mounting recess 12.
The mounting recess 12 has two side walls or abutment faces 42, 44, both of which are formed so as to have slanted faces when screwed in, the clamping screw 30 presses two sides 22, 24 of the insert 20 which have a complementary slant face, in close contact with the abutment faces 42, 44 respectively, whereby cutting edges 46 are positioned both in axial and radial directions of the cutting tool or the tool body 10. Thus, the value of deviation in the axial direction "x" and the value of deviation in the radial direction "y" of each of the cutting edges are determined.
In order to achieve a precision cutting using the aforesaid cutting tool, it is essential to maintain these deviation values "x" and "y" of each cutting edge in conformity with each other with high accuracy. Therefore, it is necessary to maintain the sides 22, 24 of the insert 20 securely in close contact with the abutment faces 42, 44 respectively. For this purpose, the cutting tool is formed in the following manner: (1) the abutment faces 42, 44 of the mounting recess 12 are precision-machined so as to secure, in predetermined accuracy, the distance .alpha..sub.1 between the abutment face 44 and the front end surface of the tool body 10 and the distance .beta..sub.1 between the abutment face 44 and the axis of rotation 0.sub.1 of the tool body 10; (2) the distances between the abutment faces 42, 44 and the center of the threaded hole 16, namely .alpha..sub.2 and .beta..sub.2 respectively, are made slightly shorter than the distances between the sides 22, 24 of the insert 20 and the center of the mounting hole 34 of the insert 20, namely .alpha..sub.3 and .beta..sub.3 respectively, so as to provide predetermined tightening margins .delta..sub.1 (=.alpha..sub.3 -.alpha..sub.2) and .delta..sub.2 (=.beta..sub.3 -.beta..sub.2) for the insert 20.
With these tightening margins, .delta..sub.1 and .delta..sub.2, the centerline O.sub.2 of the clamping screw 30 is not colinear the centerline O.sub.3 of the mounting holes 34 toward the abutment faces 42, 44, whereby the insert 20 is pressed against the abutment faces 42, 44 as the conical head 32 of the clamping screw 30 presses a tapered side 36 of the mounting hole 34 when the insert 30 is mounted. Consequently, the sides 22, 24 of the insert 20 come securely in close contact with the abutment faces 42, 44 respectively to secure the aforesaid deviation values "x" and "y".
However, conventional cutting tools with "throwaway" inserts as aforesaid have had difficulty in securing both the accuracy of machining of the abutment faces 42, 44 of the mounting recess and the accuracy for the tightening margins for the insert 20 at the same time, because of heat treatment of the tool body 10.
The tool body 10 of the aforesaid cutting tool is usually heat treated to a relatively high hardness of 43-70 on the Rockwell C hardness scale. Threaded holes 16 cannot be formed in a material of this hardness, and therefore threaded holes must be formed before the heat treatment. Thus, the position of the threaded hole 16 inevitably tends to be incorrect due to distortion caused by the heat treatment.
On the other hand, because of the necessity of removing distortions caused by heat treatments, the abutment faces 42, 44 must be specially machine-finished according to the extent of distortion following the heat treatment.
Subsequently, if the abutment faces 42, 44 are machine-finished with priority given to the distance .alpha..sub.1 between the abutment face 42 and the front end surface of the tool body 10, and the distance .beta..sub.1 between the abutment face 44 and the axis of rotation O.sub.1 of the tool body, then the distances .alpha..sub.2,.beta..sub.2 between the threaded hole 16 and the abutment faces 42, 44 become erroneous and the tightening margins .delta..sub.1,.delta..sub.2 tend to vary. Alternatively, if the abutment faces 42, 44 are machine-finished with priority given to the distances .alpha..sub.2,.beta..sub.2 between the threaded hole 16 and the abutment faces 42, 44, then the aforesaid distances .alpha..sub.1,.beta..sub.1 become erroneous and the deviation values "x" and "y" tend to vary. Consequently, in either case, the tool accuracy is adversely affected.
In order to eliminate the aforesaid disadvantage, an alternative arrangement has been introduced wherein (1) when heat treating the tool body 10, the hardness thereof is limited to a relatively low value of the Rockwell C number in the range of 30-40 so as to enabling the threaded hole 16 to be formed; (2) after heat treating, the abutment faces 42, 44 are firstly machine-finished to secure the accuracy for the distances .alpha..sub.1,.beta..sub.2 to the front end surface and to the axis of rotation O.sub.1 of the tool body 10 respectively; then (3) the threaded hole 16 is formed with reference to the finished abutment faces 42, 44 to secure the accuracy for the distances .alpha..sub.2, and .beta..sub.2 ; consequently (4) the accuracies for the aforesaid deviation values "x" and "y" and the insert tightening margins .delta..sub.1, and .delta..sub.2 are secured. This alternative, however, has the disadvantage that the hardness of the tool body 10 is absolutely insufficient, which results in remarkable damage to the tool body by chip abrasion and a shorter tool life.